X-Ray Emission Spectrum and Factors effect CH8

X-Ray Emission Spectrum

Characteristic x-ray spectrum

• X-ray emission spectra are plots of the frequency of emitted x-rays as a function of their energy.

• Characteristic x-rays have discrete energies characteristic of the differences between electron binding energies in a specific element.

• Tungsten, for example, emits x-rays with 15 discrete energies.

• For tungsten, K x-rays appear as five vertical lines on the spectrum and L x-rays as four.

• Lower energy lines correspond to emissions from outer electron shells.

• K x-rays are the only characteristic tungsten x-rays important for diagnostic radiology, and they typically represent a single vertical line at 69 keV.

Bremsstrahlung X-Ray Spectrum

• Bremsstrahlung x-rays are emitted from an x-ray tube with energies ranging from the peak electron energy to zero.
• When an x-ray tube operates at 90 kVp, bremsstrahlung x-rays up to 90 keV are emitted.
• A typical bremsstrahlung x-ray emission spectrum is displayed graphically, showing the x-ray energy on the horizontal axis and relative intensity on the vertical axis.
• The spectrum peaks at approximately one-third of the maximum energy.

Factors Affecting the X-Ray Emission Spectrum

• mA and mAs: Doubling mA doubles the number of electrons and consequently the number of x-rays emitted at each energy level, without changing the shape of the spectrum—only its amplitude.

• kVp: Increasing kVp increases the area under the emission curve (x-ray quantity), shifts the spectrum to the right (higher average x-ray energy), and keeps the maximum x-ray energy numerically equal to the kVp.

• Added Filtration: Added filtration reduces x-ray beam intensity while increasing the average energy. Low-energy x-rays are absorbed more readily than high-energy ones, causing a greater reduction on the spectrum's left side, without altering the characteristic spectrum or maximum energy.

• Target Material: The target material's atomic number (Z) affects both the quantity and effective energy (quality) of x-rays. Higher atomic number materials lead to greater bremsstrahlung efficiency and an increased number of high-energy x-rays compared to low-energy. The atomic number increase also shifts the characteristic spectrum to the right.

• Mammography Target Materials: Molybdenum (Z=42) and rhodium (Z=45) are used in mammography due to the lower x-ray quantity created from these materials.